In the manufacture of particle- and fiberboard, wood chips or fibers are normally glued with a binder, then spread out as particle mats or fiber mats and then pressed to particle or fiberboards under heat and pressure. The particle or fiberboards can be stacked hot after pressing, if needed.
Besides the acid-hardening urea-formaldehyde resins, alkali hardening phenol formaldehyde resins and adhesives based on diisocyanates as well as extracts of certain barks and woods which are combined under the category "Tannins" can be employed as the binder. Among the tannins, the so-called condensed tannins are especially suited for manufacture of particle and fiberboard.
As a rule, formaldehyde is added to the tannins as a matrix former in the manufacture of particle board. The tannins convert from the soluble state to the insoluble state by the reaction of the tannins with the formaldehyde, so that the tannin-formaldehyde resins are the actual binder.
The idea of developing adhesives based on tannin-formaldehyde resins goes back more than 30 years (Dalton 1950, 1953, Narayanamunti 1957, 1959 and Pomley et al. 1957, 1964). The cited work formed the bases for the use of tannin-formaldehyde resins as binders in wood materials. According to this work, extract material of certain barks and woods can be condensed with formaldehyde. The tannin-formaldehyde polymerizates formed from the condensation can be employed as the binder for manufacture of wood materials. Based on this technique, a few operations were able to produce plywood and particle board with tannin-formaldehyde resins contained in the binder system at various locations around the world. In the meantime, combinations of tannins with certain lignin sulfonates and sulfite spent liquors as fillers were discovered (DE-PS 3123999 A1). The combination of tannin with starch is also well known.
The application of tannin formaldehyde resins as a binder in particle boards proceeded slowly since the associated problems with application of the tannin-formaldehyde resins remained largely unsolved until now. These problems include the high viscosity of the resin in the useful consistency region (40-60% material density), relatively short drop time as a result of the high reactivity and the tendency of the tannin-formaldehyde resins to harden even before pressing in the equipment. These disadvantages, for one thing, and the nearly unlimited availability of cheap oil-based binders for another, have considerably delayed the development of these resin systems. Only the cut-back and increase in price of oil-based binders in the seventies has awakened interest in tannins and tannin-formaldehyde resins since this deals with materials recovered from secondary raw materials.
Tannins react at room temperature with formaldehyde and form tannin-formaldehyde resins. It has been shown that the tannin-formaldehyde resins have a very high viscosity in the technically relevant consistency region (40-60%) which causes problems in its preparation in normal cementing equipment. Further, uniform distribution of the binder when dispersing glue mixtures of high viscosity is not always guaranteed. The cause of the high viscosity is the reaction between the tannin and formaldehyde.